PALEOBIOLOGY: THE LATE PALEOZOIC

Table of
Contents

The Paleozoic
era literally translates as the "time of
ancient life" and spans the time period between 544 and 245 million
years ago. The Paleozoic is the first of three eras within the
Phanerozoic eon (the time of visible life). While life originated
during the Archean
and increased in complexity during the earlier Proterozoic,
the Paleozoic era is marked by the spread of animals with hard
preservable parts such as shells and exoskeletons.
This has led to what is popularly known as the Cambrian Explosion,
the sudden appearance of a stupendous array of animal life, much of
which is not closely related to modern forms. Despite extinctions at
various times, the Paleozoic is notable for the increasing
modernization of life. The late Paleozoic, the subject of this
chapter, saw the spread of plant life over the land surface and the
emergence and diversification of amphibians and their descendants the
reptiles as dominant animal life on land. Diversification of fish,
which began during the Silurian period continued unabated during the
Devonian Period. By the end of the Paleozoic, almost all major groups
of life, except the flowering plants and mammals, had developed. The
Paleozoic ended in the greatest mass
extinction event in world history. During
this massive die-off nearly 95% of all marine species went extinct.
The cause of this greatest catastrophe in Earth history has been much
investigated and scientific consensus of its cause is emerging.

During the Paleozoic we see several major advances
in life. The aformentioned Cambrian Explosion is the first. The
evolution of plants from some group of green
algae during the Ordovician is another,
since these plants moved from water onto land, paving the way for
vertebrate animals to follow. The first vertebrates, amphibians,
were little more than legged fish, although their remote descendants
would come to rule the land as reptiles,
the first truly terrestrial vertebrates.

First appearances and relative diversity (width of
shaded area) for major groups of animals. Image from Purves et
al., Life: The Science of Biology, 4th Edition, by Sinauer
Associates (www.sinauer.com) and
WH Freeman (www.whfreeman.com),
used with permission.

The fossil records of some protist and plant
groups. The width of the shaded space is an indicator of the number
of species. Image from Purves et al., Life: The Science of
Biology, 4th Edition, by Sinauer Associates (www.sinauer.com)
and WH Freeman (www.whfreeman.com),
used with permission.

The Devonian
period (410 to 360 million years ago) saw
a continued diversifiaction of life on the land, including the first
terrestrial vertebrates,
the amphibians,
and the first forests of trees. In the waters fish continued their
diversification with the rise of the lobe-finned
and ray-finned
fish. Invertebrates such as crinoids, coral, and brachiopods
thrived in shallow seas during the Devonian.

Reconstructions of Devonian life in the oceans.Top
image: Biota of a typical Devonian coral reef. Middle image:
Cladoselache fyleri, a 3-foot shark, was one of the top
predators in the Devonian seas. Bottom image: The goniatite
Goldringia is at center. Behind, the straight-shelled
cephalopod Michelinoceras is can be seen. At front left, the
trilobite Phacops is moving near a cluster of
Paraspirifer brachiopods. Images from http://seaborg.nmu.edu/earth.

Life in the Water

Invertebrates

Brachiopods continued their diversification in the
Devonian seas. One important group, the spiriferids, produced shells
with elongated hinges, as seen in the image below. The Devonian
marked the time of greatest brachiopod diversity, approximately 200
genera have been described.

Four specimens assigned to Mucrospirifer
sp. from the Middle Devonian Silica Shale of Ohio. Image from
http://www.extinctions.com,
used with permission.

Rugose and tabulate coral continued to make major
contributions to the formation of Devonian reefs. Crinoids and other
echinoderms were prominent in many fossil assemblages.

Pachyphyllum nevadense magnum from the
Martin Formation (Jerome Member) Devonian near Pine, Arizona.
Image from http://www.extinctions.com,
used with permission.

Cephalopods also underwent an increase in forms,
notably the ammonoid group known as the goniatites. These coiled,
chambered nautiloids left a great many fossils, some of which are
quite aesthetically appealing.

Michelanoceras, assorted ammonites from the
Devonian-aged Atlas Mountains Formation, Morocco.In this specimen the surrounding matrix has been cut
away and the fossils cut to reveal the inner chambers. Image
from http://www.extinctions.com,
used with permission.

The ammonoids underwent three separate
diversifications from a nautiloid-like stock. In each case the fold
pattern of sutures became more complex. These sutire patterns are
fantastic characters for identifying species, making ammonoids
excellent index fossils. The first of these occurrences was the
goniatites, a group that ranged from the Devonian to the Permian. The
ceratites are a Triassic group, while the last group, the ammonites
ranged from the Triassic
to the Cretaceous.
Ammonoids finally went extinct in the great end-of-the- Cretaceous
extinction. Nautiloids are represented today by the Nautilus.

Vertebrates

The various fish groups that had appeared during
the Silurian period (or in a few cases possibly even earlier)
continued into the Devonian. The Devonian has been called the "Age of
the Fishes" because oif the tremendous diversity of fish groups that
evolved during this period of geologic time.

Small, jawless, and finless ostracoderms were the
earliest vertebrates. They were filter feeders, but probably were
also able to move water through their gills by muscular action.
Ostracoderms have been found as fossils from the Cambrian through
Devonian periods, when the group finally went extinct. Although
extant jawless fishes lack protection, many early jawless fishes had
large defensive head shields.

The first jawed fish were the Placoderms, an
extinct group of Devonian-aged jawed fishes. Placoderms were armored
with heavy plates and had strong jaws and paired pectoral and pelvic
fins. Paired fins allow fish to balance and to maneuver well in
water, which facilitate both predation and escape.

The evolution of jaws is an example of
evolutionary modification of existing structures to perform new
functions.Jaws are modified gill arches, and
allowed the exploitation of new roles in the habitats: predators with
powerful jaws.There are two classes of jawed
fish: the cartilaginous fish and the bony fish.

Steps in the evolution of jaws by modification of
gill arches. Images from Purves et al., Life: The Science
of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com)
and WH Freeman (www.whfreeman.com),
used with permission.

The class Chondrichthyes contains approximately
850 speciesof skates, rays, and
sharks.They have jaws, lots of teeth, paired
fins, and a cartilage endoskeleton.Cartilaginous fish first appeared during the Devonian
period and expanded in diversity during the Carboniferous and Permian
before nearly disappearing during the great extinction that occurred
near the end of the Permian. Some fragmentary evidence suggests
cartilaginous fish were present during the Ordovician, although the
best skeletal evidence is from the Devonian. Despite scanty skeletal
presence (cartilage not preserving nearly so well as bone), shark
teeth are common fossils in some deposits.

The crossopterygian
fish (represented by the marine extant deep-living coelacanth and
extinct freshwater forms) are regarded as ancestors of early
amphibians.Extinct crossopterygians had
strong fins, lungs, and a streamlined body capable of swimming as
well as traveling short distances out of water.

Comparison of the skeletons of a crossopterygian
lobe-finned fish and an early amphibian. Image from Purves et
al., Life: The Science of Biology, 4th Edition, by Sinauer
Associates (www.sinauer.com) and
WH Freeman (www.whfreeman.com),
used with permission.

Life on the Land

Plants

One of the most famous fossil plant localities
that formed during the Devonian is the Rhynie
Chert from Scotland. This deposit is dated
as early Devonian and records in fine detail cells of the earlier
land vascular plants as well as other fossils. Paleogeographic
recponstructions and other evidence suggest the environment was
tropical to subtropical. Fossils from the Rynie Chert were buried in
short-lived freshwater deposits that later were subjected to
replacement of organic material with silica, forming a chert deposit
that preserved in great detail the internal cells of the fossils.
Fungi, including mycorrhizal
fungi, also have been recovered from the Rhynie Chert, suggesting the
symbiosis
of plants and fungi is indeed long and essential. Fossils assignable
to insects and crustaceans have also been recovered.

Rhynia is an early vascular plant. Like the
Silurian plant Cooksonia, Rhynia lacked leaves and
roots. One of the species formerly assigned to this genus, R.
major, has since been reclassified as Aglaophyton major.
Some paleobotanists consider A. major a bryophyte,
however, it does have a separate free-living sporophyte
that is more prominent than the gametophyte,
but appears to lack lignified
conducting cells. The remaining species, R. gwynne-vaughanii
is an undoubted vascular plant.

The Rhynie chert is one of the most famous plant
fossil localities in the world. This deposit has been of extrodinary
historical and evolutionary importance in our understanding of early
land plants and their environment. Several excellent web pages have
been developed that describe plant, algal and fungal remains from the
chert. Here, in the photo on the left, is a hand specimen of Rhynie
chert showing numerous stem-like axes and other structures in various
planes of section. On the right is a higher magnification view
showing what this matrix looks like under low magnification.
Image and text from http://lsvl.la.asu.edu/plb407/kpigg/rhyniechert.htm,
used with permission by K.B. Pigg.

Reconstruction of Aglaophyton major (A-C)
and Lyonophyton rhyniensis, another Rhynie Chert plant thought
to be the gametophyte of Aglaophyton. Image from the
UCMP Berkeley website.

The zosterophyllophytes were an important early
Devonian group that is often mentioned as ancestral to the
lycophytes. These plants had small leaf-like ennations that projected
from the stems. Ennations are not considered to be leaves since they
contain no vascular
tissue as is found in leaves.
Sporangia
in this group were not located on the ends of stems, but rather along
the sides of the stems, a feature also seen in lycophytes.
Sawdonia, shown below, a leafless Devonian plant covered with
spiny projections, is a member of this group.

Sawdonia is a member of the zosterophylls.
This group is thought to have some relationship to the lycopods,
either as lycopod ancestors or as their sister group. In this photo
you can see the numerous spiny enations that typically cover the
surface of these plants.. Image and text from http://lsvl.la.asu.edu/plb407/kpigg/sawdonia.htm,
used by permission of K.B. Pigg.

The trimeophytes were larger plants of the
early-middle Devonian. Some trimerophytes are thought ancestral to
the sphenopsids and progymnosperms of the later Devonian. Unlike the
rhyniophytes (such as Rhynia) and the zosterophyllophytes,
trimerophytes did not produce equal (dichotomous) branches. Instead
they branched in such a way that one branch appeared to be the
central stem and the other a side shoot.

Psilophyton crenulatum, dissolved from its
surrounding rock to reveal some of the three dimensional nature of
the plant. These fossils were preserved in thick mats of plant
remains and were studied in an interesting way. It was possible to
put the whole rock into a vat of acid, dissolve the rock matrix and
lift out individual plant fragments. These fragments were re-embedded
in bioplastic. Both blocks (on the left and right) show several stem
axes and, at the top, elongate paired sporangia on forked branching
systems. Image and text from http://lsvl.la.asu.edu/plb407/kpigg/pcrenulatum.htm,
used with permisson from K.B. Pigg.

The Lycophytes

Plants belonging to the division Lycophyta, have
their sporangia organized into strobili (sing.: strobilus). Leaves
that contained vascular tissue are another major advance for this
group.

Proposed steps in the evolution of the microphyll
leaf. Note that microphylls do not leave a leaf gap in the stem's
vascular cylinder. If we wanted to place Psilotum-like plants
on the left, we would have Lycopodium-like plants on the
right. Image from Purves et al., Life: The Science of
Biology, 4th Edition, by Sinauer Associates (www.sinauer.com)
and WH Freeman (www.whfreeman.com),
used with permission.

Drepanophycus is a middle Devonian
lycophyte from the Northern Hemisphere. Its features are very similar
to modern lycophytes. Sometimes this genus goes under the name
Asteroxylon.

Reconstruction of Drepanophycus , a middle
Devonian lycophyte. Note the numerous microphyll leaves, placement of
sporangia on the upper surface of the sporophylls, and stem anatomy
that are all consistent with modern lycophytes. Image from
http://www.ucmp.berkeley.edu/IB181/VPL/Lyco/Lyco2.html.

Archaeosigillaria and Colpedoxylon
are two Devonian lycopods known from areas in upstate New York. In
contrast to most lycopods which typically have simple, unforked
microphylls, these plants produce leaves with branching tips. Both
Archaeosigillaria and Colpedoxylon have leaves that
trifurcate, or have three-pointed tips. Unfortunately our material
does not show these details very well. The most complex of these
lycopods, Leclercqia, has five-forked leaves. These plants
suggest that leaves may have arisen in lycopods both by enations and
by planation of a branching system. On the left is
Archaeosigillaria, and on the right,
Colpedoxylon. Images and text from http://lsvl.la.asu.edu/plb407/kpigg/colpedoxylon.htm,
used with permission of K.B. Pigg.

The major fossil groups of lycophytes are the
lepidodendrids and sigillarids, often referred to as the arborescent
lycopods because they usually were large trees. During the middle
Devonian lycophytes retaind their herbaceous
habit, but also began to grow taller, more than a few meters high at
first, and developed the capability for secondary
growth to produce wood,
allowing the plants to grow still taller. Modern lycophytes lack
secondary growth and are entirely herbaceous.

The Sphenophytes

Once dominant elements in the Paleozoic forests,
sphenophytes (also known as the equisetophytes) are today relegated
to minor roles as stream-side herbaceous plants. The group is defined
by their jointed stems, with many extremely small leaves being
produced at a node,
production of spores in cones borne at the tips of stems, and spores
bearing elaters (devices to aid in spore dispersal). The fossil
members of this group are often encountered in coal deposits of
Carboniferous age in North America and Europe. The first sphenophytes
show up during the late Devonian, most likely evolving from some
group of the trimerophytes.

The Ferns

Ferns reproduce by spores from which the
free-living bisexual gametophyte generation develops. There are
12,000 species of ferns today, although the fossil history of ferns
shows them to have been a dominant plant group during the Paleozoic
Era. The first ferns also appear by the end of the Devonian. Some
anatomical similarities suggest that ferns and sphenophytes may have
shared a common ancestor within the trimerophytes.

The Progymnosperms

The progymnosperms are an extinct group of
free-sporing (nonseed) plants that have fern-like leaves and
reproductive structures attached to gymnosperm-like
stems with large amounts of wood. Some progymnosperms produced a
single type of spore (homospory) while otyhers produced small and
large spores, a condition known as heterospory. This latter group may
include the ancestors of seed plants.

The most famous oif the progymnosperms,
Archaeopteris, was first assigned to the ferns. During the
1960s paleobotanist Charles B. Beck convincingly demonstrated that
this common late Devonian leaf fossil was actually produced by the
same plant that also produced a common Devonian petrified wood known
as Callixylon.

The Seed Ferns

The seed ferns, fancy name pteridosperms
(literally fern-seed), appeared during the late Devonian. One of the
more important groups of seed ferns was the lyginopterids, small
vine-like plants that probably grew on other plants. This group
produced seeds in a cupule-like structure.

Animals

Perhaps the earliest animals ashore were the
invertebrates, most likely groups of arthropods. The oldest known
terrestrial arthropod is a trigonotarbid from the Silurian.
Trigonotarbids superficially looked like spiders. The major groups of
terrestrial arthropods were present during the Devonian: chelicerates
including mites, spiders, and arachnids; and made the transition to
land life, and uniramians including flightless insects, centipedes
and millipedes.

The earliest terrestrial vertebrate fossils are
from the late Devonian. These "stem tetrapods" as they are known,
were essentially fish out of water. The late Devonian lungfish had
two groups, the Dipnoi (which includes the living African,
Auistralian, and South American lungfishes) and the Crossopterygii,
whose sole living member is the coelacanth Latimeria.
Comparison of the skull, forelimb, and tooth structure of Devonian
crosspterygians such as Eusthenopteron with similar areas in a
more specialized tetrapod such as Ichthyostega supports
descent of amphibians from Devonian crossopterygians. However, some
molecular studies point to descent of the amphibians from the
lungfish, the Dipnoi.

Artist's rendition of Ichthyostega, a stem
tetrapod that may be along the lineage leading to amphibians.
Reproduced with permission. Illustration by Khristine Page (c)
1999 Exploratorium, www.exploratorium.edu.

The Devonian World

The Devonian world was one of major changes, both
in the biological realm and the geological one. The two large
landmasses, Gondwana to the south and Laurasia to the north began
moving toward the formation (again) of a world supercontinent,
Pangaea. Mountain building events were occurring on the edges of each
of these continents. On Laurasia, the collision of Eurpoe with
Laurentia produced the beginnings of the Appalachian Mountains.
Changes at the end of the Devonian brought an end to the time of
extensive reef building.

The Devonian Extinction

Near the end of the Devonian another mass
extinction occurred. This one was more severe on marine creatures
than on the newly established terrestrial forms. The corals were
quite seriously decimated, and the return of extensive reef building
did not happen until the Triassic with the evolution of a new group
of reef-building corals, the scleractinians. Brachiopods, trilobites
and primitive fish groups either were diminished or completely
snuffed out by this extinction event.

Global cooling tied to Gondwanan
glaciation has been proposed as the cause of the Devonian extinction,
as it was also suspected of causing the terminal Ordovician
extinction. Rocks in parts of Gondwana suggest a glacial event. The
forms of marine life most affected by the extinction were the warm
water to tropical ones. Another hypothesis for this extinction is
that an asteroid impact caused a global cooling. There are several
impact sites known to be of the right potential age to have been
involved in this extinction. Neither hypothesis, glaciation or
impact, is unequivocally supported by the available data.

The Carboniferous period (360 to 286 million years
ago) in Europe is better known in North America as the Mississippian
period and the Pennsylvanian period. The regression of the
Mississippian seas from North America provided a natural break in
deposition that was not echoed in Europe. In this segment the early
(or lower) Carboniferous will equate to the Mississippian, and the
late (or upper) Carboniferous will represent the Pennsylvanian.

The Carboniferous takes its name from the
widespread occurrence of coal deposits formed during this timespan in
Europe and North America. Coal is a sedimentary rock composed of
plant debris (and occasionally material from other creatues) that was
deposited in abog or swamp that had little biological activity at its
bottom. This led to preservation of the plant leaves, stems, pollen
and other structures, although with continued burial these structures
become much less distinct. Time and pressure combine with chemical
changes to turn the plant material into coal. Fluctuating sea-levels
during the Carboniferous contributed to the preservation of many coal
environments.

Life in the Water

The marine environments, following the rebound
from the late Devonian mass extinction underwent changes, with
crinoids becoming more dominant and diverse. The early Carboniferous
is sometimes known as the "age of crinoids". The primitive, armored
Devonian fish were replaced by more "modern" ray-finned fish.

Invertebrates

Fenestrate bryozoans were particularly common in
the early Carboniferous seas. Archimedes, a corkscrew-shaped
fossil, represents the secreted support of a colony of bryozoans that
are usually no longer present. The so-called lacy bryozoans
were among the first invertebrate groups to recover after the
Devonian extinction.

Brachiopods became increasingly important animals.
The spiriferids that began their diversification during the Devonian
quickly recovered from the Devonian die-off and resumed or increased
their ecological dominance during the early Carboniferous.

Corals were much restricted after the Devonian
crisis and the large reefs of the Devonian were replaced with smaller
reefs known as patch reefs. The role of corals in these new reefs was
much reduced from what it had been in earlier times. Crinoids were
also important contributors to the building of these small reefs, as
were blastoids, a similar group of echinoderms.

Forminiferans,
a group of unicellular protozoans that date from the Cambrian,
developed a new series of forms with calcareous and porcellaneous
tests (foram "shells" are technically known as tests). The fusulinids
were a large group of foraminiferans that reached sizes of several
centimeters in length. The fusulinids were very important during the
Permian before their extinction at that period's close.

Vertebrates

Sharks and bony fish continued to diversify during
the good times of the early Carboniferous, before the dropping sea
levels of the late Carboniferous caused loss of habitat. The earliest
bony fish to dominate the aquatic environment were the
palaeoniscoids, a group that raged in time from the Carboniferous to
the Triassic. Palaeoniscoids somewhat resembled modern bony fish but
had significant anatomical differences with modern fish, which belong
to a later-evolved group. The palaeoniscoids were extinct by the end
of the Mesozoic
era. Sturgeon are members of a distantly
related group to the palaeoniscoids. Sturgeon have a cartilage
skeleton and thus a poor fossil record.

Life on Land

Plants

The Lycophytes became even more significant
elements of the world's flora during the Carboniferous. These
non-seed plants evolved into trees in the fossil genera
Lepidodendron and Sigillaria, with heights reaching up
to 40 meters and 20-30 meters respectively. Lepidodendron
trunks can be over 1 meter in diameter. However, the stems are
composed of less wood (secondary xylem) that usually is found in
gymnosperm and angiosperm
trees.

We know much about the anatomy of these coal-age
lycopods because of an odd type of preservation known as a coal ball.
Coal balls can be peeled and the plants that are anatomically
preserved within them laboriously studied to learn the details of
cell structure of these coal age plants. Additionally, we have some
exceptional petrifactions and compressions that reveal different
layers of the plants' structure. Estimates place the bulk, up to 70%,
of coal material as being derived from lycophytes.

Lepidodendron was a heterosporous tree
common in coal swamps. As with many large plant fossils, one rarely
if ever finds the entire tree preserved intact. Consequently there
are a number of fossil plant genera that are "organ taxa" and
represent only the leaves (such as Lepidophylloides),
reproductive structures (Lepidostrobus), stem
(Lepidodendron), spores (Lycospora), and roots
(Stigmaria). Lepidodendron had leaves borne spirally on
branches that dichotomously forked, with roots also arising spirally
from the stigmarian axes, and both small (microspores) and large
(megaspores) formed in strobili (a loose type of soft cone). Taylor
and Taylor (1993) note that Lepidodendron reached nearly 40
meters in height, with trunks nearly 2 meters in diameter. The trees
branched extensively and produced a large number of leaves. that,
when they fell from the branches, produced the characteristic leaf
scars of the genus.

External stem features typical of arborescent
lycopods, collectively called lepidodendrids, based on the
diamond-shaped "snakeskin" type pattern produced by the helically
arranged leaf cushions. On the left is a lower magnification view of
this type of pattern, showing the general features of many of these
trees. Each leaf abscissed, so that if you are looking, as you are
here, at the outside of the stem, you can see a characteristic
appearance. On the right is a higher magnification photo showing
details of leaf cushions. Each diamond shaped cushion has a smaller
central area called the leaf base where the leaf attached. In the
center of the leaf base you can see the leaf trace, or vein to the
leaf. The vertical stripe running down each cushion is probably the
result of increased girth from secondary cortical growth inside the
stem. Images and text from
http://lsvl.la.asu.edu/plb407/kpigg/lepidodendrid.htm, used with
permission of K.B. Pigg.

Cross section through a branch (approximately an
inch in diameter) of a large lepidodendrid tree. In the very center
is a pith, surrounded by primary xylem and a small fringe of
secondary xylem [wood, MJF]. Then there is black gunk and an
open white area. Phloem and innermost cortical tissues are typically
not well preserved, and this black gunk and white areas probably
represent their positions in the branch. The outermost part of the
stem is gone. Images and text from http://lsvl.la.asu.edu/plb407/kpigg/lepidostemxs.htm,
used with permission of K.B. Pigg.

Lepidodendron was a large Carboniferous
tree reaching heights of 100 feet. When the plant's leaves fell they
left diamond-shaped scars arranged in oblique rows around the stem.
Image and text from http://seaborg.nmu.edu/earth/carbonif/car01b.html.
Note the ferns and sphenopsids growing around the
fallen Lepidodendron trunk, and a large calamite tree in the
right foreground.

Sigillaria was another arborescent lycopod,
and is also common in coals-age deposits. In contrast to the spirally
borne leaves of Lepidodendron, Sigillaria had leaved
arranged in vertical rows along the stem.

Like the lycophytes, the sphenophytes reached
their zenith during the Carboniferous and have declined to but a
single genus, Equisetum, surviving today. The largest of the
sphenophytes were trees placed in the genus Calamites.
Numerous leaf fossils, such as the specimen of Annularia shown
below, have been associated with these trees, as have reproductive
cones. Smaller shrubby plants were in the genus Sphenophyllum.

Sphenophyllum leaves of a late Cambrian
sphenopsid from the Coal Deposits of France. Image from
http://www.extinctions.com,
used with permission.

Annularia stellata, a small stem of a
Carboniferous calamite from the Coal Deposits of France. Image
from http://www.extinctions.com,
used with permission.

Ferns were well represented in the coal swamps.
Numerous fern leaf fossils have been found, often the mode of
preservation makes for striking fossils. Ferns ranged from small,
shrubby forms similar to those we see in many places today, to
tree-ferns, such as the Paleozoic genus Psaronius.

Pecopteris sp. is the foliage of a tree
fern. Pecopteris grew on Psaronius which was one of the
most common fossil tree ferns. Psaronius reached height of
about ten meters (32 feet) and resembled a modern palm tree. Although
common througout much of the late Paleozoic, this plant was most
abundant in the Late Pennsylvanian and Permian. Their increasing
abundance reflects changing climates at the close of the Paleozoic
and may be related to an increase in drier habitats. Image and
text from http://www.museum.state.il.us/exhibits/mazon_creek/pecopteris.html,
used with permission.

The seed ferns underwent increases in diversity
during the Carboniferous. Often the isolated leaf fossils are
difficult to distinguish between ferns and seed ferns. Often seed
ferns had the leaf fossil known as Neuropteris associated with
the seeds.

Neuropteris sp. is the name given to the
foliage of a seed fern. Seed ferns (Pteridospermales) are an extinct
group of gymnosperms. Although their foliage resembled that of modern
ferns, they reproduced by means of seeds. Modern ferns reproduce by
means of spores. Image and text from http://www.museum.state.il.us/exhibits/mazon_creek/neuropteris.html,
used by permission.

Other seed fern groups that characterized the
Carboniferous include the medullosans and the cordaitaleans. During
the late Carboniferous the voltzialean seed ferns appear. This latter
group is considered by paleobotanists to be closest to the
gymnosperms. Cycads,
a now-minor group of gymnosperms but major components of the world's
floras during the Mesozoic, appeared during the Carboniferous.

Specimens of Cordaites, the name-bearer for
the cordaitaleans, were large trees with strap-shaped, leathery
leaves that often had parallel leaf veins. The cordaites had their
reproductive structures and seeds arranged in cones. The cordaites
had a number of growth forms and several different types of woody
tissues have been observed in cordaite wood. The cordaites have been
interpreted as growing in an environment like the modern bald cypress
swamp.

Animals

The major animals on land during the Carboniferous
were the amphibians (and their descendants such as the stem reptiles)
and insects. Dragonfly fossils have been found with wingspans up to
75 cm.

With climate changes occurring during the late
Carboniferous, the plants changed, as did the terrestrial animals.
Glaciation in Gondwana has been blamed for this chanmge. One response
that tetrapods made was to develop the amniotic egg. With the egg and
resulting freedom from water for reproduction (to which amphibians to
this day must return to lay their eggs) reptiles (sometimes known as
stem-reptiles, could exploit drier environments further from water.

Hylonomus was one of the earliest reptiles.
This quadrupedal (four-legged) stem reptile ate insects (which were
prolific in the coal swamps). From this group of early reptiles
evolved the pelycosaurs, the great reptiles of the
Carboniferous-Permian times, and posibly the bipedal thecodonts of
the permian-Triassic. Pelycosaurs included both herbivorous and
carnovorous species. The group was characterized by an elaborate fin
or sail on their back.

The Carboniferous World

The Carboniferous, at least as seen in North
American deposits, is really two distinct sets of condiions: marine
dominated early Carboniferous environments charactrized by shallow
seas, followed in time by terrestrial "coal-swamp" environments of
the late Carboniferous. The supercontinent of Pangaea was assembled
during this time, causing the uplift of seafloor as continental land
masses collided to build the Appalachian and other mountains.

The Carboniferous Extinction

Glaciation in Gondwana during the late
Carboniferous time contributed to decline in marine environments and
fluctuating sealevels that alternately formed and destroyed coastal
coal swamp environments. There appears to have been no mass
extinction associated with this glaciation, unlike the Devonian mass
extinction.

The Permian period spanned the time interval from
286 to 245 million years ago. During the Permian the assembly of
Pangaea was completed and a whole host of new groups of organisms
evolved. The Permian ended in the greatest of the mass extinctions,
where over 90% of all species were extinguished. With the assembly of
pangaea and resulting mountain building, many of the shallow seas
retreated from the continents.

Life in the Water

The fusulinid foraminiferans that had appeard
during the Carboniferous continued their diversification. Trilobites
were rarely encountered, although brachiopods and crinoids had some
recovery of species diversity after the Carboniferous extinction.
Marine environments were much restructed due to the Pangaea-related
tectonism and resulting uplift of the the supercontinent.

Life on Land

At first glance it might appear that the only
evolutionary changes of note were occurring on the land during the
Permian. Part of this illusion results from the relative scarcity of
Permian-aged marine sediments as well as the tendency of
land-dwellers to view themselves as the pinacle of evolution.

Plant life of the Permian took on an increasingly
modern "look" with the rise of a number of gymnosperm (naked seeded)
plants during the late Carboniferous and their diversification during
the Permian. Indeed, the late Carboniferous "extinction" is almost
inapplicable to terrestrial plants. The arborescent lycopds of the
carboniferous coal swamps disappeared before the end of that period.
The Permian saw the spread of conifers and cycads, two groups that
would dominate the floras of the world until the Cretaceous period
with the rise of the flowering plants. The first conifers had small
leaves similar to those seen in the modern plant Auracaria,
the Norfolk Island pine.

Paleobotany of the Permian is also marked by an
interesting group of seed ferns completely restricted to Gondwana,
the southern parts of Pangaea. This group, the glossopterids,
produced a distinctive leaf type (classified under the leaf-genus
Glossopteris) as well as a unique, striated, bisaccate pollen
type. Glossopteris and its associated plant organs have been
found all over the former parts of Gondwana, the present continents
of India, Australia, Africa, South America and Antarctica. Seeds of
Glossopteris have been studied with computer simulations and found
too large to have travelled tyhe thousands of miles of open ocean
between Africa and South America. When Alfred Wegener proposed his
continental drift hypothesis in 1912 he used fossil evidence like
Glossopteris and its distribution to argue for the former
exstence of a supoer continent he termed Pangaea. When the bodies of
the ill-fated British expedition to the South Pole led by Captain
Scott were found in Antarctica, leaf fossils belonging to
Glossopteris were in the tents.

The amniote lineage (the first truly terrestrial
vertebrates that did not need to return to water to lay eggs),
especially the fossils, often are classified based on the number of
openings in the skulls. These openings allow for muscle attachment
and have traditionally been used to separate the "reptiles" into
several groups, including the anapsids, synapsids, and diapsids. The
anapsids, which include the turtles, lack any openings. Synapsids
include the mammals and their predecesssor and related groups the
pelycosaur (the non-mammalian synapsids). Diapsids, with two openings
on each side of their skulls, include the birds, dinosaurs, and most
of the traditional reptile groups. The synapsids used to be called
the mammal-like reptiles. Recent studies suggest that many animals in
this group were not so strongly reptilian (such as ectothermy) as the
obsolete term "mammal-like reptiles" implied.

The amniotes that had appeared during the late
Carboniferous diversified from the protothyrid stock along two
lineages: one leading to the quadrupedal pelycosaurs (stem synapsids)
of the Carboniferous-Permian and then to the therapsids (advanced
synapsids) of the Permian-Triassic; and the other leading to the
bipedal thecodonts of the Permian-Triassic. Dimetrodon, shown
above, was a member of the pelycosaurs, or non-mammalian therapsids.
By the end of the Permian the therapsids had developed, a group
possibly characterized by some degree of endothermy
(warm-bloodedness). The early mammals of the Triassic period are
possibly an offshoot or descendant group of the therapsids.

The Permian World

The Permian is geologically marked by the final
assembly of Pangaea, glaciation in the souther extreme of Gondwana,
and the greatest mass extinction in Earth history that occurred at
the close of the Permian. The Permian world was marked by a nearly
pole-to-pole supercontinent, Pangaea, that was surrounded by
Panthalassa, the world sea. A small ocean, Tethys (today represented
by the Mediterranean Sea) was also present.

The end of the Permian, also the end of the
Paleozoic era, was marked by the greatest extinction of the
Phanerozoic eon. Despite its magnitude, the terminal Permian
extinction has not received the amount of publicity or research that
the more famous, but lesser, terminal Cretaceous extinction has.
During the Permian extinction event, whose causes remain
controversial, over 95% of marine species went extinct, while 70% of
terresdtrial taxonomic familes suffered the same fate. The fusulinid
foraminiferans went completely extinct, as did the trilobites.
Brachiopod genera declined from 60 to 10. The majority of extinctions
seem to have occurred at low paleolatitudes, possibly suggesting some
event involving the ocean.

Plants seem to have missed the great extinction.
What floral changes occurred during the Permian occurred earlier in
the period, when the gradual drying out of the continent led to the
evolution and spread of better adapted "dry forms" such as
gymnosperms and seed ferns to replace the swamp trees of the
Carboniferous such as arborescent lycopods and sphenopsids. One major
plant group disappeared, the swamp gymnosperm Cordaites.

Table 1. Victims of the Permian
extinction.

fusulinid foraminifera

trilobites

rugose and tabulate corals

blastoids

acanthodians

placoderms

pelycosaurs

The cause (or causes) of the Permian extinction
remain in dispute.

The Siberian Traps are a massive lava flow in
eastern Asia that may have contributed to (or even caused) the
Permian event by triggering a massive, sudden glaciation as well
as other environmental consequences of volcanic eruptions. The
period of greatest eruption in the traps coincides with the mass
extinction. Ages of the lava flows suggest the traps formed over a
one million year interval. The traps are near the city of Tura and
also occur in Yakutsk, Noril'sk and Irkutsk, covering an area of
slightly less than 2 million square kilometers (an area larger
than Europe). The original volume of the traps is estimated at
between 1 million and 4 million cubic km.

The Cretaceous-Tertiary extinction that marks
the close of the Mesozoic era has been hypothesized as a result of
a large meteor or comet strike. Evidence of a similar (and larger)
impact at the close of the Permian is not strongly supported,
although some indirect evidence suggests an impact did occur
during the Permian, although possibly not at the time of the
extinction crisis.

Climate change, possibly caused by glaciation
and/or volcanic activity, has been associated with many mass
extinctions. It seems likely that climate change is a consequence
of the cause of extinction rather than the root cause itself.

Formation of Pangaea has been invoked as a
cause for the extinction. However, the formation of Pangaea had
occurred well before the mass extinction. The mountain building
and associated tectonism would also have been extremely long term
in duration, possibly 10 million years according to some studies
of the snowball garnets from Vermont. Pangaea's presence may have
led to extreme environments with hotter interior areas of the
continent and colder polar areas, possibly producing glaciation.

There is evidence of a sudden drop in sea
level at the end of the Permian. This could be attributed to the
sudden cooling of the climate that produced glaciers at the
southern polar areas in Gondwana. Glacial tillites and other
geological evidence of late Permian age occur in Australia,
Siberia and in the North Sea are interpreted as proof of the
existence of large continental glaciers at the close of the
Permian.

Poisoning of the ocean has been suggested due
to an apparent drop in carbon isotope data obtained from marine
sediments formed at the time of the extinction. The cause of this
apparent dropoff in the photosynthetic rate in the seas has not
yet been determined.

Whatever cause (or combination of causes), the
terminal Permain extinction was a massive and severe crisis for life.
Many groups of organisms went extinct at that time. Surviving groups
diversified during the Triassic period and gradually a more modern
world developed.

Fossils,
Rocks, and Time This online version of a U.S. Geological
Survey general interest publication, by Lucy E. Edwards and John
Pojeta, Jr., gives additional insight to application of the study
of fossils.

A
Geologic Timeline This site gives the majopr events in Earth
hostory presented against twelve standard hours, from midnight to
noon.

Geologic
Ages of Earth History This site by Jeff Poling give a much
more detailed geologic time scale than is commonly encountered in
introductory textbooks. Rest assured, as you get deeper into any
field of science, that things will most assuredly get more
detailed!

The
Major Fossil Groups This U.S. Geological Survey page provides
additional details about selected groups of fossil-producing
organisms.

ORTHOCERAS
& AMMONITES This commercial page offers a clear discussion
and tasty illustrations of thsi important group of fossil
cephalopods.

The Fish Out of
TimeTM This commercial website from dinofish.com
offers video clips and much much more about the mysterious "living
fossil", the coelacanth.

Devonian
Times Presented with a newspaper type style, this wonderful
site offers fossils as well as reconstructions of the Devonian
biota recovered from Pennsylvania.

Coal
Information This webpage from the Kentucky Geological Survey
furnishes us with information about what is coal, and how coal
forms.

A
HISTORY OF PALAEOZOIC FORESTS This page by Hans Kerp of the
UNIVERSITY of MÜNSTER was originally in German in Natur
und Museum ( 1996, Vol. 126, No. 12, pp. 421 - 430 ), but is
now in English, and provides you with a starting point for a
virtual paleobotanical tour of the Paleozoic.

The
Fossil Cycads This online article provides a nicley
illustrated look at these important Mesozoic plants.